The invention relates to an apparatus for conveying collapsed plastic films.
Known apparatus for conveying plastic films with a guiding system work satisfactorily only when the plastic films, extruded, for example, as a continuous tube, have certain properties. These include, in particular, a smooth surface and a small thickness, which does not disadvantageously affect the flexibility of the continuous film, so that the continuous film, such as, in particular, a collapsed, continuous blown film can be passed in an orderly manner under changing horizontal angles of approach and departure about the turning bars, which usually are held in the rack of the apparatus in such a manner that they do not rotate. It follows from this that particularly those plastic films, which have an external dull or tacky surface, are not suitable for being handled in the known guiding systems. Examples of such films are rubber-like films or collapsed films or blown films, which are produced by the coextrusion method, the one or external layer of which consists, for example, of an ethylene vinyl acetate copolymer (EVA), while the other or internal layer of which is formed from a non-tacky or smooth plastic in the form of a conventional polyolefin such as polyethylene. Such films are used, for example, in the case of a blown film, after the collapsed, continuous blown film has been slit, as surface-protection films, which are glued with their tacky side onto the objects that are to be protected. However, plastic films, which have a greater thickness or are made from a material that imparts stiffness, collapsed, continuous films of which are difficult to guide around the turning bars, are included among those, which are unsuitable for the known guiding systems.
In all of these cases, the continuous film experiences strains and stresses to such an extent at the turning bar, that it cannot be passed through the guiding system without being damaged. This can also not be remedied in the case of turning bars constructed as so-called air turning bars, by intensifying more and more the cushion of air produced by the usual row of air nozzles, since the guiding accuracy and the fold-free and deformation-free course of the continuous film are lost in this manner.
The problems described show themselves particularly during the manufacture of continuous plastic films by the blowing method, in which permissible variations in the thickness of the film wall arise, for example, because of the usual permissible manufacturing variations in the blown film die of the extruder used to produce the film, in the calibration equipment traversed after the extrusion process by the continuous blown film with expanded circular cross section, as a result of the internal and external cooling or as a result of other external circumstances. When such continuous blown films are wound up in the collapsed state in reeling equipment, annular tings would be formed in the reeled film due to the addition of permissible variations in the thickness, unless countermeasures are taken. These annular rings would result in a lasting deformation of the film in this region and would make it difficult to print on the continuous blown film and to process this film further.
So-called reversing take-off units have been developed for the continuous blown film as suitable countermeasures to prevent the formation of annular rings on the reeled film. When the continuous blown film is collapsed, the reversing system of the take-off unit causes the fold edges, which form as the continuous blown film is collapsed, to migrate periodically. This brings about a corresponding distribution of any permissible variations in thickness over the width of the collapsed blown film. During the subsequent reeling of the continuous collapsed film in reeling equipment, the formation of annular rings on the reel due to the addition of permissible variations in the thickness of the film is avoided in this manner. It is self-evident that a sufficiently large pivoting angle must be chosen for the reversing motion.
In the case of a known, reversing take-off unit of the German patent 20 35 584, the collapsing apparatus, comprising the usual nip roller rotatable about horizontal axes, reversing through an angle of about 360.degree., is swiveled about the axis of the continuous blown film supplied in a horizontal plane. It has been observed that this pivoting angle of a reversing motion is appropriate for the bulk of the applications of a reversing take-off unit. The reversing system comprises two guide rollers and two turning bars for deflecting the continuous blown film through an angle of about 180.degree. while at the same time changing the direction. On the other hand, in those application cases, in which the pivoting angle of the reversing motion of the collapsing unit with its nip roller is less than 360.degree., a single turning bar between two guide rollers or also in conjunction with only one guide roller may also be adequate for handling the continuous blown film in the reversing system of the take-off unit.
In the case of the known take-off unit of the German patent 20 35 584, the support for the guide rollers and the turning bars, which can be swiveled about the axis of the continuous blown film supplied, is produced in the apparatus in such a manner, that the axes of the guide rollers and of the turning bars run tangentially to the circles around the vertical axis of rotation of the collapsing unit, the guide rollers, in every possible operating position, being radially outside of the parts of the turning bars looped by the continuous blown film. The pivoting angles of the guide rollers and the turning bars decrease as the distance from the collapsing unit increases. The continuous blown film is guided in mutually parallel horizontal planes between the guide rollers and the turning bars and the stationary conveyor for transporting the collapsed, continuous film away. By these means, a small overall height of the reversing system is attained. Furthermore, the measure that the radius of the circle circumscribed by the pivoting path of the axes of the turning bars amounts to .pi./4.times. the diameter of the turning bars, counteracts any lateral running of the continuous blown film during the pivoting of the reversing system. At the same time, it is found that the intersection of these center lines of the continuous blown film strands running towards and away from the turning bar lies in the central axis of rotation of the reversing system. During the reversing pivoting motion, it is thus ensured that the continuous blown-film strands open and close in scissors fashion about the axis of rotation so that, theoretically, identical symmetrically opposite fictional forces act on the two strands of continuous blown film and, by the very concept, axial shifting of the continuous blown film on the turning bars during the reversing operation is avoided as a result of the frictional forces acting on one side.